The present disclosure relates generally to hermetic barrier layers, and more particularly to methods and compositions used to seal solid structures using low melting temperature glasses.
Hermetic barrier layers can be used to protect sensitive materials from deleterious exposure to a wide variety of liquids and gases. As used herein, “hermetic” refers to a state of being completely or substantially sealed, especially against the escape or entry of water or air, though protection from exposure to other liquids and gases is contemplated.
Glass-to-glass bonding techniques can be used to sandwich a workpiece between adjacent substrates and generally provide a degree of encapsulation. Conventionally, glass-to-glass substrate bonds such as plate-to-plate sealing techniques are performed with organic glues or inorganic glass frits. Device makers of systems requiring thoroughly hermetic conditions for long-term operation generally prefer inorganic metal, solder, or frit-based sealing materials because organic glues (polymeric or otherwise) form barriers that are generally permeable to water and oxygen at levels many orders of magnitude greater than the inorganic options. On the other hand, while inorganic metal, solder, or frit-based sealants can be used to form impermeable seals, the resulting sealing interface is generally opaque as a result of the metal cation composition, scattering from gas bubble formation, and distributed ceramic-phase constituents.
Frit-based sealants, for instance, include glass materials that have been ground to a particle size ranging typically from about 2 to 150 microns. For frit-sealing applications, the glass frit material is typically mixed with a negative CTE material having a similar particle size, and the resulting mixture is blended into a paste using an organic solvent. Example negative CTE inorganic fillers include cordierite particles (e.g. Mg2Al3[AlSi5O18]) or barium silicates. The solvent is used to adjust the viscosity of the mixture.
To join two substrates, a glass frit layer can be applied to sealing surfaces on one or both of the substrates by spin-coating or screen printing. The frit-coated substrate(s) are initially subjected to an organic burn-out step at relatively low temperature (e.g., 250° C. for 30 minutes) to remove the organic vehicle. Two substrates to be joined are then assembled/mated along respective sealing surfaces and the pair is placed in a wafer bonder. A thermo-compressive cycle is executed under well-defined temperature and pressure whereby the glass frit is melted to form a compact glass seal.
Glass frit materials, with the exception of certain lead-containing compositions, typically have a glass transition temperature greater than 450° C. and thus require processing at elevated temperatures to form the barrier layer. Such a high-temperature sealing process may be detrimental to temperature-sensitive workpieces.
Further, the negative CTE inorganic fillers, which are used in order to lower the thermal expansion coefficient mismatch between typical substrates and the glass frit, will be incorporated into the bonding joint and result in a frit-based barrier layer that is neither transparent nor translucent.
Based on the foregoing, it would be desirable to form glass-to-glass seals at low temperatures that are transparent and optionally hermetic.